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Evolution of surface morphology of NiO thin films under swift heavy ion irradiation
Applied Surface Science 256 (2009) 521–523
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evolution of surface morphology of NiO thin films under swift heavy ion irradiation P. Mallick a, Chandana Rath b, S. Majumder c, R. Biswal d, D.C. Agarwal e, Shikha Varma c, D.K. Avasthi e, P.V. Satyam c, N.C. Mishra d,* a
Department of Physics, North Orissa University, Baripada 757003, India School of Material Science & Technology, Institute of Technology, BHU, Varanasi 221005, India Institute of Physics, Bhubaneswar 751005, India d Department of Physics, Utkal University, Vani Vihar, Bhubaneswar, Orissa 751004, India e Inter-University Accelerator Center, P.O. Box 10502, New Delhi 110067, India b c
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 5 August 2009
NiO nanoparticle thin films grown on Si substrates were irradiated by 107 MeV Ag8+ ions. The films were characterized by glancing angle X-ray diffraction and atomic force microscopy. Ag ion irradiation was found to influence the shape and size of the nanoparticles. The pristine NiO film consisted of uniform size (100 nm along major axis and 55 nm along minor axis) elliptical particles, which changed to also of uniform size (63 nm) circular shape particles on irradiation at a fluence of 3 1013 ions cm 2. Comparison of XRD line width analysis and AFM data revealed that the particles in the pristine films are single crystalline, which turn to polycrystalline on irradiation with 107 MeV Ag ions. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.80.Jh 61.82.Rx 61.82.Ms 68.37.Ps 68.55.Ac Keywords: Ion irradiation Nanoparticles Atomic force microscopy NiO
1. Introduction An energetic ion traversing through materials medium transfers its energy mainly by two nearly independent processes: nuclear (elastic) energy loss (Sn) and electronic (inelastic) energy loss (Se). The Sn induced process dominates in the keV range of ion energy, which leads to creation of atomic size point defects and clusters of defects. For swift heavy ion (SHI) moving at velocity comparable to the Bohr velocity of the electron, the Se induced process is the dominant mechanism for energy transfer and materials modification [1]. This inelastic collision process leads to a coherent excitation and ionization of electrons along the ion path. Subsequent transfer of energy from the electrons to the lattice atoms can lead to such exotic effects like sputtering of target material, production of defects, surface and interface morphological changes, etc. [2] in different classes of materials which cannot be generated by any other means. When the Se exceeds a threshold value, Seth, a permanent signature of the passage of the ions in the
* Corresponding author. Tel.: +91 674 2581079; fax: +91 674 2581079. E-mail address: [email protected] (N.C. Mishra). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.107
bulk material is registered in the form of amorphized latent tracks [3,4]. The track diameter is about few nanometers and length can be tens to hundreds of micrometers. When the dimension of a material is reduced to nanoscale, the response of material to SHI irradiation can be very different from that of the bulk. Thus literature abounds with such conflicting reports as enhanced irradiation resistance of nanoparticles [5], fragmentation and evaporation of nanoparticles [6] and even growth of nanoparticles under SHI irradiation [7,8]. In this paper we report modification of surface morphologies of NiO nanoparticle thin films under 107 MeV Ag8+ ion irradiation. We show that both shape and size of the nanoparticles undergo a considerable modification under SHI irradiation. The single crystalline nanoparticles in the pristine film acquire polycrystalline nature at high fluences of irradiation. 2. Experimental 200 nm thick NiO thin films were deposited on Si substrates by ebeam evaporation technique from a target made out of NiO in pressed pellet form as described elsewhere [8]. The films were deposited at ambient substrate temperature and the film deposition
522
P. Mallick et al. / Applied Surface Science 256 (2009) 521–523
rate was 0.2 nm s 1. The as deposited films were irradiated at room temperature (RT) with 107 MeV Ag ions from the 16 MV tandem Pelletron accelerator at IUAC, New Delhi. Irradiation was performed in the direction perpendicular to the sample surface in a slightly offnormal condition (58 with respect to surface normal). The ion beam was scanned over a 1 cm 1 cm area covering the complete sample surface for uniform irradiation. The samples were mounted on a copper target ladder using silver paste. The target ladder was placed inside the high vacuum chamber (10 6 Torr) during irradiation. The structural characterization of the pristine as well as irradiated films was done by glancing angle X-ray diffraction (GAXRD) at room temperature under identical conditions. A Brucker X-ray diffractometer (Model D8) with Cu Ka radiation source (l = 1.54 A˚), flat LiF monochromator and scintillation detector in the u–2u geometry at a fixed 38 angle of incidence on the sample was used for the GAXRD study. Go¨bel mirror and fixed slit (2 mm) in the incident beam side, and long soller slit in the reflected beam side were used. The u–2u scans were performed using a step size of 0.058 and a dwell time of 3 s per step. The full width at half maximum (FWHM) of X-ray peaks and the peak positions were measured by Lorentzian fit of XRD peaks. Evolution of surface morphology with irradiation fluence was investigated using atomic force microscope (AFM) (Digital instruments, Nanoscope IIIa). 3. Result and discussion The Se, Sn and range (RP) of the 107 MeV Ag ions in NiO as estimated from SRIM-2006 simulation programme [9] are about 27, 0.16 keV nm 1 and 7.5 mm, respectively. Since the RP of the ion is much larger than the film thickness, ions are implanted much deeper in the substrate and ion energy is deposited uniformly in the film along the ion path. Since Se is much larger than Sn, the observed modification in the NiO matrix is mainly due to the Se induced processes. Further the Se is less than the threshold electronic energy loss, Seth for creating continuous tracks in bulk NiO (30 keV nm 1 [10]). In nanoparticles, the modifications induced by irradiation however can be very different from that of the bulk [5–8]. We present below the modifications induced by 107 MeV Ag ion irradiation in nanoparticle thin films of NiO. The XRD patterns of the pristine and irradiated films (Fig. 1) reveal that the fcc structure of NiO is retained at all fluences of irradiation. The lattice parameter (0.418 nm) obtained from the XRD pattern also did not change with irradiation fluence. SHI irradiation is known to induce considerable strain in the bulk crystalline materials [11]. Absence of XRD peak shift in our case
Fig. 2. Williamson–Hall (W–H) plot for the pristine as well as 107 MeV Ag ion irradiated NiO films. The inset shows the rms value of strain fluctuation and crystallite size obtained from the W–H plot.
indicated absence of macro-strain. The XRD peak however broadened with ion fluence (Fig. 1). The full width at half maximum (FWHM) of the Bragg peaks at different ion fluences can have contributions from root mean square (rms) value of strain fluctuation and the finite crystallite size. These parameters extracted from the Williamson–Hall (W–H) analysis [12] of the FWHM (b) of various Bragg peaks (Fig. 2), are given in Table 1. The rms value of strain fluctuation remains almost same with increasing fluence up to 1 1012 ions cm 2, and then shows an increase at higher fluences with a peak at the fluence of 3 1012 ions cm 2. The crystallite size on the other hand showed a monotonic decrease with increasing irradiation fluence except for a slight increase at the fluence where the strain had a maximum value. AFM images of the surface morphologies of pristine and 107 MeV Ag ion irradiated NiO thin films on Si substrates at different fluences are shown in Fig. 3. The irradiated as well as pristine films are granular in nature. Irradiation changed the shape and size of the NiO nanoparticles. The pristine NiO film consisted of elliptical shape nanoparticles. The shape of most of the nanoparticles changed from elliptical to circular on irradiation at fluence of 3 1011 ions cm 2. A few nanoparticles were still elliptical at this fluence. At a fluence of 1 1012 ions cm 2, all particles acquire circular shape. The shapes of the nanoparticles remain circular up to the highest fluence (3 1013 ions cm 2) of irradiation. Unlike the average crystallite size calculated from the XRD line widths, the size of the particle obtained from AFM did not show a dramatic change (Table 1). The elliptical particles of the pristine film had an average dimension of 100 nm along major axis and 55 nm along minor axis. Considering the area of an elliptical shape particle in terms of its equivalent spherical shape, one can find the average diameter of the particles in the pristine film as 74 nm. The average Table 1 The rms value of strain fluctuation and crystallite size variation with ion fluences as obtained from XRD line width analysis. The particle size at various ion fluences obtained from AFM analysis is also given for comparison. Fluence (ions cm
Fig. 1. The XRD pattern of pristine and 107 MeV Ag ion irradiated NiO thin films.
0 3 1011 1 1012 3 1012 1 1013
2
)
Williamson–Hall analysis
AFM analysis
Strain
Crystallite size (nm)
Particle size (nm)
0.006 0.007 0.006 0.02 0.012
72.8 56.5 26.4 37.3 13.3
74 60 46 67 69
P. Mallick et al. / Applied Surface Science 256 (2009) 521–523
Fig. 3. 1 mm 1 mm 2D AFM images of: (a) pristine film, film irradiated with (b) 3 1011 ions cm (f) 3 1013 ions cm 2.
particle size decreased to 60 nm on irradiating the films at the fluence of 3 1011 ions cm 2. Irradiation at a higher fluence (1 1012 ions cm 2) led to further decrease of particle size to 46 nm. At still higher fluence (3 1012 ions cm 2), the average particle size increased to about 67 nm and again decreased at the highest fluence. Though the average particle size did not show a monotonic variation with ion fluence, ion irradiation influenced the distribution of particle size considerably. At the highest fluence (3 1013 ions cm 2), for example we observe well dispersed and almost uniform circular size (63 nm) NiO nanoparticles. Comparing the particle size as obtained from AFM and the crystallite size as obtained from XRD line width analysis at different ion fluences (Table 1), we note that the particles in the pristine film are single crystalline. Increasing ion fluence caused a large decrease in the crystallite size, whereas the particle size showed a small variation with irradiation fluence. Thus at high ion fluences, the particles develop polycrystalline nature and the number of crystallites in a particle increases with increasing ion fluence. 4. Conclusion We report modification of surface morphologies of NiO nanoparticle thin films on Si substrates by 107 MeV Ag ion irradiation. The fcc structure of NiO is retained at all fluences of irradiation and the lattice parameter also did not change with irradiation fluence. The shape and size of the nanoparticles, however, changed considerably with irradiation fluence. Elliptical nanoparticles of size 100 nm along major axis and 55 nm along minor axis in the pristine film changed to almost uniform size (63 nm) circular particles at the fluence of 3 1013 ions cm 2. The particles in the pristine film were single crystalline, which
2
, (c) 1 1012 ions cm
523
2
, (d) 3 1012 ions cm
2
, (e) 1 1013 ions cm
2
and
developed polycrystalline nature on irradiation. The number of crystallites in a particle increased with increasing irradiation fluence. Acknowledgements The authors are thankful to the Pelletron group of IUAC, New Delhi, for providing a good quality scanned beam for irradiation. The authors are thankful to Prof. S.N. Sahu and Dr. S. Sarangi, IOP, Bhubaneswar for providing XRD facility. R. Biswal would like to acknowledge CSIR, Government of India for SRF (F. No. 09/173/ (0126)/2008/EMR-I). References [1] R.L. Fleischer, P.B. Price, R.M. Walker, J. Appl. Phys. 36 (1965) 3645. [2] S. Chandramohan, R. Sathyamoorthy, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Asokan, V. Ganesan, J. Mater. Sci.: Mater. Electron. 18 (2007) 1093. [3] A. Barbu, A. Dunlop, D. Lesueur, R.S. Averback, Europhys. Lett. 15 (1991) 37. [4] J.M. Costantini, F. Studer, J.C. Peuzin, J. Appl. Phys. 90 (2001) 126. [5] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, J. Mag. Mag. Mater. 186 (1998) 342. [6] A. Berthelot, S. Hemon, F. Gourbilleau, C. Dufour, E. Dooryhee, E. Paumier, Nucl. Instrum. Methods Phys. Res. B 146 (1998) 437. [7] D. Mohanta, S.S. Nath, A. Bordoloi, A. Choudhury, S.K. Dolui, N.C. Mishra, J. Appl. Phys. 92 (2002) 7149; D. Mohanta, N.C. Mishra, A. Choudhury, Mater. Lett. 58 (2004) 3694. [8] P. Mallick, D.C. Agarwal, C. Rath, R. Biswal, D. Behera, D.K. Avasthi, D. Kanjilal, P.V. Satyam, N.C. Mishra, Nucl. Instrum. Methods Phys. Res. B 266 (2008) 3332. [9] http://www.srim.org/. [10] B. Schattat, W. Bolse, S. Klaumu¨nzer, I. Zizak, R. Scholz, Appl. Phys. Lett. 87 (2005) 173110. [11] B. Roas, B. Hensel, S. Henke, S. Klaumu¨nzer, B. Kabius, W. Watanabe, G. SaemannIschenko, L. Schultz, K. Urban, Europhys. Lett. 11 (1990) 669. [12] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Evolution of surface morphology of NiO thin films under swift heavy ion irradiation P. Mallick a, Chandana Rath b, S. Majumder c, R. Biswal d, D.C. Agarwal e, Shikha Varma c, D.K. Avasthi e, P.V. Satyam c, N.C. Mishra d,* a
Department of Physics, North Orissa University, Baripada 757003, India School of Material Science & Technology, Institute of Technology, BHU, Varanasi 221005, India Institute of Physics, Bhubaneswar 751005, India d Department of Physics, Utkal University, Vani Vihar, Bhubaneswar, Orissa 751004, India e Inter-University Accelerator Center, P.O. Box 10502, New Delhi 110067, India b c
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 5 August 2009
NiO nanoparticle thin films grown on Si substrates were irradiated by 107 MeV Ag8+ ions. The films were characterized by glancing angle X-ray diffraction and atomic force microscopy. Ag ion irradiation was found to influence the shape and size of the nanoparticles. The pristine NiO film consisted of uniform size (100 nm along major axis and 55 nm along minor axis) elliptical particles, which changed to also of uniform size (63 nm) circular shape particles on irradiation at a fluence of 3 1013 ions cm 2. Comparison of XRD line width analysis and AFM data revealed that the particles in the pristine films are single crystalline, which turn to polycrystalline on irradiation with 107 MeV Ag ions. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.80.Jh 61.82.Rx 61.82.Ms 68.37.Ps 68.55.Ac Keywords: Ion irradiation Nanoparticles Atomic force microscopy NiO
1. Introduction An energetic ion traversing through materials medium transfers its energy mainly by two nearly independent processes: nuclear (elastic) energy loss (Sn) and electronic (inelastic) energy loss (Se). The Sn induced process dominates in the keV range of ion energy, which leads to creation of atomic size point defects and clusters of defects. For swift heavy ion (SHI) moving at velocity comparable to the Bohr velocity of the electron, the Se induced process is the dominant mechanism for energy transfer and materials modification [1]. This inelastic collision process leads to a coherent excitation and ionization of electrons along the ion path. Subsequent transfer of energy from the electrons to the lattice atoms can lead to such exotic effects like sputtering of target material, production of defects, surface and interface morphological changes, etc. [2] in different classes of materials which cannot be generated by any other means. When the Se exceeds a threshold value, Seth, a permanent signature of the passage of the ions in the
* Corresponding author. Tel.: +91 674 2581079; fax: +91 674 2581079. E-mail address: [email protected] (N.C. Mishra). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.107
bulk material is registered in the form of amorphized latent tracks [3,4]. The track diameter is about few nanometers and length can be tens to hundreds of micrometers. When the dimension of a material is reduced to nanoscale, the response of material to SHI irradiation can be very different from that of the bulk. Thus literature abounds with such conflicting reports as enhanced irradiation resistance of nanoparticles [5], fragmentation and evaporation of nanoparticles [6] and even growth of nanoparticles under SHI irradiation [7,8]. In this paper we report modification of surface morphologies of NiO nanoparticle thin films under 107 MeV Ag8+ ion irradiation. We show that both shape and size of the nanoparticles undergo a considerable modification under SHI irradiation. The single crystalline nanoparticles in the pristine film acquire polycrystalline nature at high fluences of irradiation. 2. Experimental 200 nm thick NiO thin films were deposited on Si substrates by ebeam evaporation technique from a target made out of NiO in pressed pellet form as described elsewhere [8]. The films were deposited at ambient substrate temperature and the film deposition
522
P. Mallick et al. / Applied Surface Science 256 (2009) 521–523
rate was 0.2 nm s 1. The as deposited films were irradiated at room temperature (RT) with 107 MeV Ag ions from the 16 MV tandem Pelletron accelerator at IUAC, New Delhi. Irradiation was performed in the direction perpendicular to the sample surface in a slightly offnormal condition (58 with respect to surface normal). The ion beam was scanned over a 1 cm 1 cm area covering the complete sample surface for uniform irradiation. The samples were mounted on a copper target ladder using silver paste. The target ladder was placed inside the high vacuum chamber (10 6 Torr) during irradiation. The structural characterization of the pristine as well as irradiated films was done by glancing angle X-ray diffraction (GAXRD) at room temperature under identical conditions. A Brucker X-ray diffractometer (Model D8) with Cu Ka radiation source (l = 1.54 A˚), flat LiF monochromator and scintillation detector in the u–2u geometry at a fixed 38 angle of incidence on the sample was used for the GAXRD study. Go¨bel mirror and fixed slit (2 mm) in the incident beam side, and long soller slit in the reflected beam side were used. The u–2u scans were performed using a step size of 0.058 and a dwell time of 3 s per step. The full width at half maximum (FWHM) of X-ray peaks and the peak positions were measured by Lorentzian fit of XRD peaks. Evolution of surface morphology with irradiation fluence was investigated using atomic force microscope (AFM) (Digital instruments, Nanoscope IIIa). 3. Result and discussion The Se, Sn and range (RP) of the 107 MeV Ag ions in NiO as estimated from SRIM-2006 simulation programme [9] are about 27, 0.16 keV nm 1 and 7.5 mm, respectively. Since the RP of the ion is much larger than the film thickness, ions are implanted much deeper in the substrate and ion energy is deposited uniformly in the film along the ion path. Since Se is much larger than Sn, the observed modification in the NiO matrix is mainly due to the Se induced processes. Further the Se is less than the threshold electronic energy loss, Seth for creating continuous tracks in bulk NiO (30 keV nm 1 [10]). In nanoparticles, the modifications induced by irradiation however can be very different from that of the bulk [5–8]. We present below the modifications induced by 107 MeV Ag ion irradiation in nanoparticle thin films of NiO. The XRD patterns of the pristine and irradiated films (Fig. 1) reveal that the fcc structure of NiO is retained at all fluences of irradiation. The lattice parameter (0.418 nm) obtained from the XRD pattern also did not change with irradiation fluence. SHI irradiation is known to induce considerable strain in the bulk crystalline materials [11]. Absence of XRD peak shift in our case
Fig. 2. Williamson–Hall (W–H) plot for the pristine as well as 107 MeV Ag ion irradiated NiO films. The inset shows the rms value of strain fluctuation and crystallite size obtained from the W–H plot.
indicated absence of macro-strain. The XRD peak however broadened with ion fluence (Fig. 1). The full width at half maximum (FWHM) of the Bragg peaks at different ion fluences can have contributions from root mean square (rms) value of strain fluctuation and the finite crystallite size. These parameters extracted from the Williamson–Hall (W–H) analysis [12] of the FWHM (b) of various Bragg peaks (Fig. 2), are given in Table 1. The rms value of strain fluctuation remains almost same with increasing fluence up to 1 1012 ions cm 2, and then shows an increase at higher fluences with a peak at the fluence of 3 1012 ions cm 2. The crystallite size on the other hand showed a monotonic decrease with increasing irradiation fluence except for a slight increase at the fluence where the strain had a maximum value. AFM images of the surface morphologies of pristine and 107 MeV Ag ion irradiated NiO thin films on Si substrates at different fluences are shown in Fig. 3. The irradiated as well as pristine films are granular in nature. Irradiation changed the shape and size of the NiO nanoparticles. The pristine NiO film consisted of elliptical shape nanoparticles. The shape of most of the nanoparticles changed from elliptical to circular on irradiation at fluence of 3 1011 ions cm 2. A few nanoparticles were still elliptical at this fluence. At a fluence of 1 1012 ions cm 2, all particles acquire circular shape. The shapes of the nanoparticles remain circular up to the highest fluence (3 1013 ions cm 2) of irradiation. Unlike the average crystallite size calculated from the XRD line widths, the size of the particle obtained from AFM did not show a dramatic change (Table 1). The elliptical particles of the pristine film had an average dimension of 100 nm along major axis and 55 nm along minor axis. Considering the area of an elliptical shape particle in terms of its equivalent spherical shape, one can find the average diameter of the particles in the pristine film as 74 nm. The average Table 1 The rms value of strain fluctuation and crystallite size variation with ion fluences as obtained from XRD line width analysis. The particle size at various ion fluences obtained from AFM analysis is also given for comparison. Fluence (ions cm
Fig. 1. The XRD pattern of pristine and 107 MeV Ag ion irradiated NiO thin films.
0 3 1011 1 1012 3 1012 1 1013
2
)
Williamson–Hall analysis
AFM analysis
Strain
Crystallite size (nm)
Particle size (nm)
0.006 0.007 0.006 0.02 0.012
72.8 56.5 26.4 37.3 13.3
74 60 46 67 69
P. Mallick et al. / Applied Surface Science 256 (2009) 521–523
Fig. 3. 1 mm 1 mm 2D AFM images of: (a) pristine film, film irradiated with (b) 3 1011 ions cm (f) 3 1013 ions cm 2.
particle size decreased to 60 nm on irradiating the films at the fluence of 3 1011 ions cm 2. Irradiation at a higher fluence (1 1012 ions cm 2) led to further decrease of particle size to 46 nm. At still higher fluence (3 1012 ions cm 2), the average particle size increased to about 67 nm and again decreased at the highest fluence. Though the average particle size did not show a monotonic variation with ion fluence, ion irradiation influenced the distribution of particle size considerably. At the highest fluence (3 1013 ions cm 2), for example we observe well dispersed and almost uniform circular size (63 nm) NiO nanoparticles. Comparing the particle size as obtained from AFM and the crystallite size as obtained from XRD line width analysis at different ion fluences (Table 1), we note that the particles in the pristine film are single crystalline. Increasing ion fluence caused a large decrease in the crystallite size, whereas the particle size showed a small variation with irradiation fluence. Thus at high ion fluences, the particles develop polycrystalline nature and the number of crystallites in a particle increases with increasing ion fluence. 4. Conclusion We report modification of surface morphologies of NiO nanoparticle thin films on Si substrates by 107 MeV Ag ion irradiation. The fcc structure of NiO is retained at all fluences of irradiation and the lattice parameter also did not change with irradiation fluence. The shape and size of the nanoparticles, however, changed considerably with irradiation fluence. Elliptical nanoparticles of size 100 nm along major axis and 55 nm along minor axis in the pristine film changed to almost uniform size (63 nm) circular particles at the fluence of 3 1013 ions cm 2. The particles in the pristine film were single crystalline, which
2
, (c) 1 1012 ions cm
523
2
, (d) 3 1012 ions cm
2
, (e) 1 1013 ions cm
2
and
developed polycrystalline nature on irradiation. The number of crystallites in a particle increased with increasing irradiation fluence. Acknowledgements The authors are thankful to the Pelletron group of IUAC, New Delhi, for providing a good quality scanned beam for irradiation. The authors are thankful to Prof. S.N. Sahu and Dr. S. Sarangi, IOP, Bhubaneswar for providing XRD facility. R. Biswal would like to acknowledge CSIR, Government of India for SRF (F. No. 09/173/ (0126)/2008/EMR-I). References [1] R.L. Fleischer, P.B. Price, R.M. Walker, J. Appl. Phys. 36 (1965) 3645. [2] S. Chandramohan, R. Sathyamoorthy, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Asokan, V. Ganesan, J. Mater. Sci.: Mater. Electron. 18 (2007) 1093. [3] A. Barbu, A. Dunlop, D. Lesueur, R.S. Averback, Europhys. Lett. 15 (1991) 37. [4] J.M. Costantini, F. Studer, J.C. Peuzin, J. Appl. Phys. 90 (2001) 126. [5] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, J. Mag. Mag. Mater. 186 (1998) 342. [6] A. Berthelot, S. Hemon, F. Gourbilleau, C. Dufour, E. Dooryhee, E. Paumier, Nucl. Instrum. Methods Phys. Res. B 146 (1998) 437. [7] D. Mohanta, S.S. Nath, A. Bordoloi, A. Choudhury, S.K. Dolui, N.C. Mishra, J. Appl. Phys. 92 (2002) 7149; D. Mohanta, N.C. Mishra, A. Choudhury, Mater. Lett. 58 (2004) 3694. [8] P. Mallick, D.C. Agarwal, C. Rath, R. Biswal, D. Behera, D.K. Avasthi, D. Kanjilal, P.V. Satyam, N.C. Mishra, Nucl. Instrum. Methods Phys. Res. B 266 (2008) 3332. [9] http://www.srim.org/. [10] B. Schattat, W. Bolse, S. Klaumu¨nzer, I. Zizak, R. Scholz, Appl. Phys. Lett. 87 (2005) 173110. [11] B. Roas, B. Hensel, S. Henke, S. Klaumu¨nzer, B. Kabius, W. Watanabe, G. SaemannIschenko, L. Schultz, K. Urban, Europhys. Lett. 11 (1990) 669. [12] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22.